This application incorporates by reference of Taiwan application Ser. No. 90106122, filed on Mar. 15, 2001.
1. Field of the Invention
The invention relates in general to an apparatus for thermal detection, and more particularly to an apparatus for detecting temperatures of fluid inside a cavity device.
2. Description of the Related Art
Most inkjet printers now use thermal inkjet print heads to eject ink droplets onto a sheet of medium, such as paper, for printing. The thermal inkjet print head includes ink, heating devices, and nozzles. The heating devices heat the ink to create bubbles until the bubbles expand enough so that ink droplets through the nozzles are fired onto the sheet of paper to form dots. Varying the sizes and locations of the ink droplets can form different texts and graphics on a sheet of paper.
The thermal inkjet technology and resolution of an inkjet printer determine the printing quality that the inkjet printer can provide. Currently, entry-level color printers provide a maximum resolution of 720 by 720 dot per inch (dpi) or 1440 by 720 dpi. The size of the droplets is related to the surface tension and viscosity of the ink, and finer size of the droplets provides higher printing resolution. As to the thermal inkjet technology, a print head structure disclosed in U.S. Pat. No. 6,102,530 to Kim, et al., is shown in FIG. 1. In order to fabricate a print head 100, a structure layer 120 is first formed on a semiconductor substrate, such as silicon wafer 140, and then a manifold 150 and a chamber 130 are formed by anisotropic etching on the silicon wafer 140. After that, ink ejectors are gradually formed and each of the ink ejectors includes a first heater 160, a second heater 165, and a nozzle 110, as shown in FIG. 1. Arrays of the ink ejectors are arranged on the print head 100 so as to eject ink 190. Since each structure of the ink ejectors is identical in practice, only a few ink ejectors are illustrated in FIG. 1 for the sake of brevity. As shown in FIG. 1, the nozzle 110 is disposed above the chamber 130 and the chamber 130 is adjacent to and in flow communication with the manifold 150. Thus, the ink 190 from a reservoir (not shown) fills each chamber 130 by passing through the manifold 150, and the ink 190 is allowed to be ejected via each nozzle 110. Note that each nozzle 110 is equipped with heaters, such as the first heater 160 and second heater 165, for heating the corresponding chamber 130 in order to increase the temperature of the ink 190 in the chamber 130. When the temperature of the ink 190 in the chamber 130 rises, bubbles are formed therein and expand correspondingly. The bubbles expand so that ink droplets are forced to be ejected via the nozzle 110 onto a printing medium. In the following, the forming process of the ink droplets is described.
FIG. 2 is a cross-sectional view of the print head 100 in FIG. 1. In FIG. 2, the first heater 160 and second heater 165 are disposed around the nozzle 110. The two heaters heat up so as to form bubbles 210 and 215. The bubbles 210 and 215 expand in the direction of arrows P as the two heaters continue to heat up, and the ink 190 in the chamber 130 is pressurized, thus it causes the ink 190 to be ejected through the nozzle 110 as an ink droplet in direction F, as shown in FIG. 2.
In brief, if a specific nozzle such as the nozzle 110 is desired to eject ink droplets, the heaters 160 and 165 disposed around the nozzle 110 are activated to heat the ink 190 in the associated chamber 130 to form bubbles 210 and 215 so as to eject ink droplets from the nozzle 110 onto a printing medium. Note that the ink 190 in the chamber 130 can reach a temperature greater than a maximum level, for example, after the nozzle 110 was used for ink ejection for a period of time. In this case, if the ink 190 at the high temperature is still heated by the heaters 160 and 165 and they are supplied with the same power used in the normal situation, the ink 190 overheats and the viscosity of the ink 190 is lowered, resulting in the degradation of the printing quality. Conversely, the ink 190 in the chamber 130 can reach a temperature smaller than a minimum level, for example, after the nozzle 110 was inactive for ink ejection for a period of time. For the ink 190 at the low temperature, if the power applied to the heaters 160 and 165 does not increase and is not greater than that used in the normal situation, the ink 190 will not reach a required temperature and ink droplets will be failed to be ejected. Thus, in order to maintain a good quality of printing, the ink 190 in the chambers 130 should be controlled within a predetermined range.
Accordingly, the technique for detecting the temperature of ink and performing thermal compensation in response to the detected temperature is important to the printing quality. An approach to the detection of the temperature of the ink is described in U.S. Pat. No. 5,696,543, xe2x80x9cRecording head which detects temperature of an element chip and corrects for variations in that detected temperature, and cartridge and apparatus having such a headxe2x80x9d to Koizumi, et al. In this approach, a chip employs a resistor as a temperature sensor, and an adjusting resistor used outside the chip to form a temperature detecting circuit in the form of Wheatstone bridge circuitry. This approach has the disadvantages of its complexity in detection and high production cost so that it is not suitable for mass production. Therefore, some other temperature detecting device that has better sensitivity, reduced complexity, and a low production cost is needed.
It is therefore an object of the invention to provide a piezo-resistive thermal detection apparatus for detecting the temperature of fluid inside a cavity device so that the fluid temperature is capable of being controlled within a predetermined range with heaters, such as annular heaters, thus enabling the improvement in the printing quality.
The invention achieves the above-identified object by providing a piezo-resistive temperature detection apparatus including a detection region and a plurality of piezo-resistive devices, for detecting the temperature of fluid inside a cavity device, such as an inkjet print head. For an inkjet print head, in practice, its ink temperature can be controlled within a predetermined operating thermal range by using heaters disposed around the edges of the print head. The detection region, for example a rectangular detection region made of semiconductor material, is formed on the print head. The piezo-resistive devices, for example resistors made of polysilicon, are disposed on the centers of edges of the detection region, wherein the piezo-resistive devices change their resistances in response to the deformation of the piezo-resistive devices because of stresses exerted on them. When the ink temperature rises, the surface that the detection region is disposed on (i.e., the surface of the print head) protrudes, resulting in the deformation of the piezo-resistive devices. The resistances of the piezo-resistive devices thus change because of the stresses exerted on the piezo-resistive devices. The piezo-resistive devices, such as resistors, can be connected together in the form of a circuit bridge, such as Wheatstone bridge circuitry, so that a voltage signal indicative of the changes in the resistances of the piezo-resistive devices can be outputted. In this way, the ink temperature can be obtained, based on the voltage signal outputted. In order to enhance the gauge factor of the piezo-resistive devices and thus produce a larger detection signal, the piezo-resistive devices can be doped with such as boron or phosphorous ions during manufacturing process of the piezo-resistive devices. In addition to polysilicon, the piezo-resistive devices can be made of metal, such as a material selected from the group consisting of aluminum, gold, copper, tungsten, titanium, tungsten nitride, titanium nitride, and alloys of aluminum-silicon-copper.
Other objects, features, and advantages of the invention will become apparent from the following detailed description of the preferred but non-limiting embodiments. The following description is made with reference to the accompanying drawings.